This invention relates to a gas refining method using a pressure swing adsorption (PSA) method and particularly to a PSA method for refining high purity carbon monoxide from a mixed gas containing a low concentration of carbon monoxide.
The PSA method is such that a specific component gas contained in a material gas is adsorbed by adsorbent by increasing a pressure, and the pressure is then reduced to separate the specific component gas from the material gas by desorbing the specific component from the adsorbent. The above process is described in detail with respect to an exemplary case where the specific component gas is a carbon monoxide gas. The above process is comprised of 1 a pressure increasing step of increasing a pressure inside an adsorption column charged with adsorbent to a specified adsorption pressure, 2 an adsorbing step of adsorbing carbon monoxide by the adsorbent by admitting a material gas containing carbon monoxide into the adsorption column, 3 a pressure decreasing step of decreasing the pressure inside the adsorption column to or near an atmospheric pressure after the adsorbing step, 4 a cleaning step of discharging an exhaust gas from the adsorption column by supplying a part of the high purity carbon monoxide gas collected as a gas product to the adsorption column, and 5 a product collecting step of desorbing the carbon monoxide adsorbed by the adsorbent by decreasing the pressure inside the adsorption column below the atmospheric pressure and collecting the desorbed carbon monoxide as a gas product. By repeating the above steps, carbon monoxide can be refined from the material gas containing carbon monoxide. For the separation of the carbon monoxide gas, an adsorbent in which cuprous chloride is supported on a carrier such as activated alumina, activated carbon, and synthetic zeolite is used.
In actually performing operations using the PSA method, it is a general practice to install a plurality of adsorption columns one after another to continuously collect a gas product so as to increase a production efficiency. For example, in the case that four adsorption columns A to D are installed as shown in FIG. 1, operations run in accordance with an operation flow chart of FIG. 2.
Material gases used to refine a carbon monoxide gas therefrom according to the PSA method include converted gases of by-product gases produced at iron works, methanol cracked gases, and gases reformed from naphtha, LNG, LPG, or the like. The carbon monoxide gas products collected from these material gases are expected to have a purity of 99% or higher. The production of such high purity carbon monoxide gas is relatively easy in the case that the high purity converted gas having a carbon monoxide concentration of 70% or higher is used. It is, however, difficult to collect a high purity gas product from a methanol cracked gas having a carbon monoxide concentration of about 30%. In view of this difficulty, for example, Japanese Unexamined Patent Publication No. 7-136444 discloses a PSA method for collecting a high purity gas product by admitting a gas used to increase a pressure into the adsorption column in a direction opposite from a direction in which the material gas is admitted during the pressure increasing step.
Recently, there has been a demand to collect a high purity carbon oxide gas product from a reformed gas of very low carbon monoxide concentration of about 10%. It is possible to collect the high purity carbon monoxide gas product according to the PSA method using a reformed gas as a material gas. However, this necessitates a considerable increase in the amount of the gas product admitted into the adsorption column during the cleaning step, resulting in a reduction in the amount of the collected gas product. An attempt to ensure a specified amount of gas product necessitates a considerable increase in the amount of adsorbent. Accordingly, there is a demand for a method for increasing an amount of collected gas product by improving the PSA method.